1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor light emitting device suitably used for a light emitting diode (LED), a laser diode (LD), an electronic device etc., and a group III nitride compound semiconductor light emitting device, and a lamp.
Priority is claimed on Japanese Patent Application No. 2006-315497, filed Nov. 22, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Since a group III nitride semiconductor light emitting device has a band gap of a direct transition type of energy corresponding to a range from visible light to ultraviolet rays, and excels in luminous efficiency, a group III nitride semiconductor light emitting device is used as a light emitting device such as an LED or an LD.
In addition, in the case in which a group III nitride semiconductor is used for an electronic device, it provides electronic devices having characteristics superior to those derived from the conventional group III-V compound semiconductors.
Such a group III nitride compound semiconductor is, in general, produced from trimethyl gallium, trimethyl aluminum and ammonia as raw materials through a Metal-Organic Chemical Vapor Deposition (MOCVD) method. The MOCVD method is for growing a crystal by introducing a vapor of a raw material into a carrier gas to convey the vapor to the surface of a substrate and decomposing the raw material using a reaction with the heated substrate.
Hitherto, a single crystal wafer of a group III-V compound semiconductor is, in general, produced by growing a crystal onto a single crystal wafer of a different material. There is a large lattice mismatching between such a different kind of substrate and a group III nitride semiconductor crystal which grows epitaxially thereon. For example, in the case in which gallium nitride (GaN) was grown on a sapphire (Al2O3) substrate, there is a lattice mismatching of 16% therebetween, and in the case in which gallium nitride was grown on a SiC substrate, there is a lattice mismatching of 6% therebetween.
In general, there is a problem in that it is possible to grow a crystal epitaxially on a substrate when there is a lattice mismatching as in the above, and that it is impossible to provide a crystal having excellent crystallinity even if it is grown.
Thus, a method which includes laminating a layer which is called a low temperature buffer layer consisting of aluminum nitride (AlN) or aluminum nitride gallium (AlGaN) on a substrate, and then epitaxially growing a group III nitride semiconductor crystal thereon, as a method for epitaxially growing a group III nitride semiconductor crystal on a sapphire single crystal substrate or a SiC single crystal substrate has been disclosed and widely performed, as shown in, for example, Patent document 1 (Japanese Patent Publication No. 3,026,087) and Patent document 2 (Japanese Unexamined Patent Application, First Publication No. 4-297023).
In addition, a method which is called an Facing-Target cathode of facing a target to another target, for forming an AlN layer has been proposed in, for example, a Non-patent Document 1 (Kikuo Tominaga et al., “Japanese Journal of Applied Of Physics”, Vol. 28, p7 (1989)).
In addition, a method for forming an AlN layer on a substrate using a DC magnetron sputtering method has been proposed in, for example, Non-patent Document 2 (M Ishihara et al., “Thin Solid Film)”, vol. 316, p152 (1998))).
In addition, as a method which includes forming an AlN layer as a barrier layer by a method other than the MOCVD method, and forming another layer thereon by a MOCVD method, a method which includes forming a buffer layer using a high frequency sputtering, and growing a crystal having the same composition using a MOCVD method thereon has been proposed in for example Patent Document 3 (Japanese Examined Application, Second Publication No. 5-86646).
However, there is a problem in that an excellent crystal cannot be stably produced by the method disclosed in Patent Document 3.
Thus, in order to stably produce an excellent crystal, for example, a method which includes annealing in a mixed gas consisting of an ammonia and hydrogen after a buffer layer is grown has been proposed in for example Patent Document 4 (Japanese Patent Publication No. 344o873), and a method for forming a buffer layer by using a DC sputtering at a temperature of not less than 400° C. has been proposed in for example Patent document 5 (Japanese Patent Publication No. 3700492).
However, since the methods disclosed in Patent Documents 4 and 5 form a buffer layer by a sputtering method, the resultant buffer layer may have poor crystallinity in terms of the conditions necessary for forming a layer, although the rate of forming layer is high. When a GaN layer is grown up onto a buffer layer with such a low crystallinity by MOCVD method, there is possibility that the crystallinity of GaN layer significantly deteriorates.
On the other hand, research has been conducted on producing a group III nitride compound semiconductor crystal by sputtering. A method of forming a GaN layer onto a (100) face of Si and a (0001) face of Al2O3 by high frequency magnetron sputtering using N2 gas has been proposed in, for example, Non-patent Document 3 (Y. USHIKU et al., “the 21st century alliance symposium memoirs”, Vol. 2nd, p295 (2003)).
In addition, a method for forming a GaN layer using an apparatus in which a cathode faces a solid target and a mesh which is inserted between a substrate and the target has been proposed in for example Non-patent Document 4 (T. Kikuma et al., “Vacuum”, Vol. 66, P233 (2002)).
In addition, when a GaN layer consisting of a group III nitride compound semiconductor crystal in the above is formed, for example, it is necessary to layer a crystal in which dopant elements such as Si or Mg have been doped.
In such a case, a target in which dopant is mixed into a Ga metal which serves as a mother material, and a GaN layer is formed by a sputtering method, as shown in for example Non-patent Document 5 (Japan Society of Applied Physics “No. 66 Japan Society of Applied Physics” pamphlet, 7a-N-6 (autumn, 2005), p248)).
However, there is a problem in that it is difficult to minutely adjust the doping ratio when dopant is mixed with a Ga metal by the method disclosed in Non-patent document 5.
In addition, there is a problem in that it is difficult for Mg to enter when Al composition is high, since in the case in which an AlGaN using Mg dope as a group III nitride layer is layered, the amount of Mg taken into a crystal corresponding to Al composition, although Mg will easily enter when Al composition is low.
In addition, there is a problem in that the apparatus becomes complex and the processing time is long. Another problem is that the yield to the target amount deteriorates, when Ga is mixed with a dopant clement in the sputtering apparatus using the method disclosed in Non-patent Document 5.
For these reasons, a method has been requested for stably forming a GaN layer with excellent crystallinity at a high efficiency, being capable of optimizing the composition of the target material which includes Ga and the mixing balance of dopant concentration, using an apparatus having a simple constitution.
The present invention was made in view of the above problems, and it is an object of the present invention to provide a process for producing a group III nitride compound semiconductor light emitting device, the group III nitride compound semiconductor light emitting device and a lamp, the process being capable of mixing Ga serving as a mother material, a mixed crystal and a dopant element at a proper balance, and forming a stable crystal film consisting of a group III nitride compound semiconductor with excellent uniformity at a high efficiency for a short period using an apparatus having a simple constitution, excellent productivity and excellent light emitting characteristics.